Variable geometry turbine

ABSTRACT

A variable geometry turbine in which a turbine wheel is mounted to rotate about a pre-determined axis within a housing. A sidewall is displaceable relative to a surface of the housing to control the width of a gas inlet passage defined adjacent the wheel between the sidewall and the housing surface. The sidewall is supported on rods extending parallel to the wheel rotation axis, and the rods are displaced to control the displacement of the sidewall relative to the housing. The housing defines a chamber into which the rods extend such that one or more piston and cylinder arrangements are defined. The pressure within the chamber is controlled to control the axial position of the piston, the sidewall being displaced as a result of displacement of the piston.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a variable geometry turbineincorporating a displaceable turbine inlet passage sidewall.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,522,697 describes a known variable geometry turbine inwhich a turbine wheel is mounted to rotate about a pre-determined axiswithin a housing. An inlet passage to the turbine wheel is definedbetween a fixed wall of the housing and a sidewall which is displaceablerelative to the fixed wall in order to control the width of an inletpassage. The sidewall is supported on rods extending parallel to thewheel rotation axis, and the rods are axially displaced relative to thehousing so as to control the position adopted by the sidewall.

The rods are displaced by a pneumatic actuator mounted on the outside ofthe housing, the pneumatic actuator driving a piston. The actuatorpiston is coupled to a lever extending from a shaft pivotally supportedby the housing such that displacement of the lever causes the shaft toturn. A yoke having two spaced apart arms is mounted on the shaft in acavity defined within the housing. The end of each arm of the yoke isreceived in a slot in a respective sidewall support rod. Displacement ofthe actuator piston causes the arms to pivot and to drive the sidewallin the axial direction as a result of the interengagement between thearms and the sidewall support rods.

The known variable geometry turbine exhibits various disadvantageousfeatures. In particular, pneumatic actuators typically incorporate anelustomeric diaphragm which is prone to failure, particularly in thetemperature, piston stroke and pressure environment associated withvariable geometry turbines. The shaft which supports the yoke is exposedto high temperatures but cannot be readily lubricated and therefore wearcan arise. Furthermore, the engagement of the levers with the rods is ofa sliding nature and although it is known to incorporate wear resistantmaterials, e.g. ceramics, in such assemblies, wear can still be aproblem. Finally, mounting a pneumatic actuator outside the housingincreases the overall size of the assembly which can be a criticalfactor in some applications.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate one ormore of the problems outlined above.

According to the present invention, there is provided a variablegeometry turbine comprising a housing, a turbine wheel mounted to rotateabout a pre-determined axis within the housing, a sidewall which isdisplaceable relative to the housing to control the width of a gas inletpassage defined adjacent the wheel between a first surface defined bythe sidewall and a second surface defined by the housing, anddisplacement control means for controlling displacement of the sidewallrelative to the housing, wherein the housing defines at least onechamber forming a cylinder which receives a piston defined by thesidewall, the sidewall is displaced as a result of displacement of thepiston, and the displacement control means comprise means forcontrolling the pressure within the said at least one chamber to controlthe position of the sidewall relative to the housing.

The piston and cylinder may be annular.

The sidewall may be supported on guide rods extending parallel to thewheel rotation axis. The sidewall and guide rod assembly may be biasedaway from or towards the second surface by at least one spring. Each rodmay be biased by one or more springs. The spring or springs may have avariable spring rate such that the rate of change of spring force withgas inlet passage width increases as the sidewall approaches the secondsurface. For example, each guide rod may be acted upon by two springs,one spring being compressed only when the sidewall approaches thehousing surface.

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an upper half of a sidewall assembly of avariable geometry turbine in accordance with the present invention, thesidewall being shown in a position in which a gas inlet passageway is ofminimum width;

FIG. 2 shows the lower half of the sidewall assembly of FIG. 1 with thesidewall displaced to the fully open position;

FIGS. 3 and 4 show alternative spring arrangements for the sidewallsupport rods shown in FIGS. 1 and 2; and

FIG. 5 is a schematic representation of characteristics of the springassembly of FIG. 4 and the reactant gas force and resultant force on thesidewall of FIG. 4.

FIG. 6 is a sectional view representing an alternative control assemblyfor a sidewall support rod.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIGS. 1 and 2, the illustrated variable geometry turbinecomprises a housing formed by a bearing housing 1 and a turbine wheelhousing 2 clamped together with an annular clip 3, and a turbine wheel 4mounted on a shaft 5 to rotate about an axis 6. The shaft 5 is supportedon bearings within the bearing housing 1. The turbine housing 2 definesa surface 7 facing a surface 8 defined by a sidewall 9. The sidewall 9in the illustrated assembly is shown formed from relatively thin steeland in cross-section is generally C-shaped, but it will be appreciatedthat the sidewall 9 could be for example a cast component. Vanes 10mounted on the sidewall project from the surface 8 into an annularrecess 11 defined in the housing. A sidewall which supports vanes as inthe illustrated assembly is sometimes referred to as a “nozzle ring”,but the term “sidewall” will be used herein.

Sealing rings 12 prevent gas flow between an inlet passageway 13 definedbetween the surfaces 7 and 8 and a chamber 14 located on the side of thesidewall remote from the vanes 10. Thus the sidewall 9 forms an annularpiston received within an annular cylinder that defines the chamber 14.Support rods 15 on which the sidewall 9 is mounted extend into thechamber 14. An inlet 16 is formed in the bearing housing 1 to enablecontrol of the pressure within the chamber 14. Increasing that pressuremoves the sidewall 9 towards a fully closed position shown in FIG. 1,whereas reducing that pressure moves the sidewall 9 towards a fully openposition as shown in FIG. 2.

Thus, the pressure within the chamber 14 is used to control the axialdisplacement of the sidewall 9. Means (not shown) are provided forcontrolling the pressure within the chamber 14 in accordance with acontrol program responsive to for example engine speed and torque andturbine pressures and temperature. The pressure control means is coupledto the inlet 16.

Referring to FIG. 3, this illustrates one arrangement forspring-mounting the support rods 15 in the bearing housing 1. In thearrangement shown in FIG. 3, which corresponds to the sidewall 9 ofFIGS. 1 and 2 in the fully open position, each support rod extendsthrough a bore in the bearing housing 1 into a cavity 17. The cavity 17is defined between the bearing housing 1 and a further housing component18 coupled to the bearing housing 1. The pressure within cavity 17 ismaintained close to atmospheric pressure.

The rod 15 is biased towards the left in FIG. 3 by a compression spring19 compressed between the bearing housing 1 and a washer 20 retained onthe end of the rod 15. Thus if the chamber 14 is vented to atmosphere,the rod 15 will assume the axial position shown in FIG. 3. If thepressure within the chamber 14 is then increased, the rod 15 andsidewall 9 will be displaced towards the right in FIG. 3 by a distancedependent upon the applied pressure.

Referring now to FIG. 4, components equivalent to those described inFIG. 3 carry the same reference numerals. In the arrangement of FIG. 4however it will be noted that a further compression spring 21 which iscoaxial with the axis 6 bears against an annular support ring 22 whichperforms the same function as the washers 20 in the arrangement of FIG.3. Each support rod 15 also extends through a coaxial compression spring19. Thus the force driving the rod 14 to the left in FIG. 4 is thecombination of the compression forces applied by the springs 19 and 21,and any axial forces applied to the sidewall 9 by the gas flowingthrough the inlet passage 13.

The springs 19 and 21 are arranged such that the return force applied tothe rods 15 increases as the surface 8 of the sidewall 9 approaches thesurface 7 defined by the turbine housing 2. For example, the spring 21may have a length when in its relaxed state such that it does not opposemovement of the ring 22 to the right in FIG. 4 except when the sidewall9 is relatively close to the surface 7. It has been found that this isan advantageous characteristic as the pressure within the inlet passage13, which pressure acts on the surface 8, reduces as the surface 8approaches the surface 7 due to the flow conditions within the gapdefined between those two surfaces.

FIG. 5 illustrates the operational differences between an arrangementsuch as that described with FIG. 3, in which the spring 19 has a linearspring rate, and the arrangement of FIG. 4 in which the combination ofsprings 19 and 21 provides a non-linear spring rate. In FIG. 5, thecurves represent axial forces applied to the assembly of componentsincluding the sidewall 9 as the distance between the surfaces 7 and 8(the inlet passage width) is increased from a minimum 23 (fully closedas shown in FIG. 1) to a maximum 24 (fully open as shown in FIG. 2).

Curve 25 of FIG. 5 represents the variation of axial force due toreactant gas forces on the surface 8 of the sidewall 9. It will be notedthat as the passage width is reduced the reactant gas force initiallyrises in a substantially linear fashion but then falls as the sidewall 9approaches the surface 7 of the turbine housing 2. The curves 26 and 27represent the force applied by the spring 19 of FIG. 3. The curves 28and 29 represent the resultant axial force on the sidewall 9, theresultant force reducing with reduction in passage width beyond thedistance indicated by line 30. Thus with an arrangement as shown in FIG.3 in which the springs 19 have linear characteristics, the axialposition of the sidewall 9 is unstable when the inlet passage width isreduced to the limit represented by line 30. In particular, there willbe a tendency for the sidewall to be moved rapidly to the minimum widthposition in an uncontrolled manner as soon as is passes the positionrepresented by line 30.

With the arrangement of FIG. 4, the spring 21 has no effect when theinlet passage width is in the range represented by the distances betweenthe lines 24 and 31. As soon as the passage width is reduced to thelimit represented by line 31 however, further reductions in the passagewidth compress both the spring 21 and the springs 19. As a result thecombined spring characteristic is as represented by lines 26 and 32, andthe resultant is represented by lines 28 and 33. Thus the resultant ofthe spring and reactant gas forces increase continuously as the inletpassage width reduces to the minimum represented by line 23. Instabilityin the axial position of the sidewall 9 is thus avoided.

Referring to FIG. 6, the same reference numerals are used as in FIGS. 1to 4 however, rather than the chamber 14 and the sidewall 9 defining apiston and cylinder arrangement, each rod 15 is coupled to an annularpiston 34 which supports sealing rings 35 such that pressure within thechamber 17 on the side of the piston 34 remote from the spring 19indirectly controls the axial position of the rods 15 by controlling theaxial position of the ring 34. The differential pressure across thepiston 34 is controlled by controlling the pressure within a control airinlet 36. The pressure on the spring side of piston 34 is maintainedclose to atmospheric.

With the arrangement of FIG. 6, apertures (not shown) may be providedthrough the sidewall 9 to open into face 8 and thereby reduce the forcedifferential across the sidewall as described in U.S. Pat. No.5,522,697. Such an arrangement is not possible if the cavity immediatelybehind the sidewall is used as a control cylinder as in the case of thearrangements of FIGS. 1 to 4.

In some circumstances, it is desirable to bias the sidewall to a fullyclosed position, rather than towards a fully open position as in thearrangements of FIGS. 1 to 4 and 6. This could be achieved by placingthe springs 19 shown in FIG. 6 on the left of the piston 34 rather thanon the right, and positioning the control pressure inlet 36 tocommunicate with the right hand end of the cavity 17.

It will also be appreciated that although the moveable sidewall 9 ispositioned in the bearing housing 1 of the illustrated arrangements, thesidewall could be supported in the turbine housing 2 by reversing thelocations of the relevant components with respect to the inlet passage13. This would make it possible to achieve cost reductions by using acommon bearing housing 1 for both fixed and variable geometry turbines.

The present invention provides various advantages as compared with theknown variable geometry turbine. Firstly, as no actuator mechanicallycoupled to the sidewall is required, the problems associated with suchactuators are avoided. Secondly, as mechanical couplings between anactuator and the sidewall have been eliminated, potential points of wearare also eliminated. This could be achieved by placing the springs 19shown in FIG. 6 on the left of the piston 34 rather than on the right,and positioning the control pressure inlet 36 to communicate with theright hand end of the cavity 17.

Having thus described the invention, what is claimed as novel and desired to be secured by Letters Patent of the United States is:
 1. A variable geometry turbine comprising a housing, a turbine wheel mounted to rotate about a pre-determined axis within the housing, a sidewall which is displaceable relative to the housing to control the width of a gas inlet passage defined adjacent the wheel between a first surface defined by the sidewall and a second surface defined by the housing, and displacement control means for controlling displacement of the sidewall relative to the housing, wherein the housing defines at least one chamber forming an annular cylinder which receives a piston comprising an annular member coupled to and defined by the sidewall, the sidewall is displaced as a result of displacement of the piston, and the displacement control means comprise means for controlling the pressure within the said at least one chamber to control the position of the sidewall relative to the housing said sidewall being supported on guide rods parallel to the wheel rotation axis, said guide rods being biased by at least one spring away from the second surface.
 2. A variable geometry turbine according to claim 1, wherein each rod is biased by at least one spring away from the second surface.
 3. A variable geometry turbine according to claim 2, wherein the said at least one spring has a variable spring rate such that the rate of change of spring force with gas flow passage width increases as the sidewall approaches the second surface.
 4. A variable geometry turbine according to claim 3 wherein each rod extends through a respective compression spring bearing against the housing and the rod, and a further compression spring is arranged to bear against the end of each rod, the said further spring being compressed only when the sidewall approaches the second surface. 